Antimicrobial Titanium–Copper Alloys: The Role of Microstructure in Arc-Melted Compositions

Abstract

Copper-containing alloys have attracted global attention in response to rising rates of orthopedic implant infections and antimicrobial resistance. Two theories have emerged for the antimicrobial mechanisms of titanium–copper alloys: ion release and contact sterilization. While previous studies have focused on the overall effect of intermetallic Ti₂Cu phases, this research digs deeper, unpicking the influence of precipitate size and morphology. Herein, heat treatment (950 °C and 760/820 °C) of cast Ti-11.5Cu and Ti-33Cu (wt%) alloys may be used as a tool to tune antimicrobial potency through microstructural refinement. Specifically, it is shown that nanoscale precipitates (≈30 nm) of Ti₂Cu exhibit limited in vitro efficacy against Staphylococcus aureus. While larger (≈5 μm), rounded precipitates exhibit superior action due to increased surface contact. Contrary to the literature, this study shows no detectable Cu²⁺ ion release in 0.9% NaCl over 7 days, measured by inductively coupled plasma optical emission spectroscopy, suggesting that under these manufacturing conditions, the antimicrobial mechanism is solely contact dependent. In vitro studies indicate that while Ti₂Cu phases contribute to antimicrobial properties, a balance in Cu content and precipitate size is critical for both bacterial and native bone cell cytotoxicity. Overall, this study demonstrates a significant link between phase size, morphology, and desirable antimicrobial properties of exploratory cast Ti–Cu alloys.